1. Field of the Invention
This invention relates to woven papermakers' fabrics and, particularly, to woven fabrics for the wet end of the paper machine.
2. Description of the Prior Art
In the course of the developent of the technology relating to papermakers' fabrics twill weave patterns came to be preferred to a plain weave because the twill patterns by virtue of their surface floats provided superior sheet support and by virtue of less yarn interlacing through the center of the fabric afforded a more crowded or tighter weave structure and, hence, better fiber retention. However, the conventional twill weave patterns proved unacceptable because the fabric lacked the requisite structural stability. This lack of structural stability is due to the fact that adjacent yarns are sufficiently locked in place by crimping to retain their intended position within the weave pattern and tend to be shoved together in service, a fabric defect sometimes referred to in the art as "shoviness."
The so-called "reverse broken twill" (RBT) weave patterns have gained wide acceptance in the art as a compromise between the superior sheet support and fabric dewatering afforded by a twill pattern and the fabric stability afforded by a plain weave. FIG. 1 of the drawings illustrates a conventional reverse broken twill woven on 4 harnesses (an example of a four end repeat). Viewing that weave pattern, it can be appreciated that it affords a plurality of surface two-floats in combination with the interlocking afforded by warp yarns 2 and 4 woven as in a plain weave. However, the reverse broken twill of FIG. 1 presents the art with another problem which is referred to herein as "unequal warp sew-up." Bearing in mind the maxim the the shortest distance between two points is a straight line, it can be seen that warps 2 and 4 in the reverse broken twill pattern of FIG. 1 require a greater length of yarn in that pattern than do yarns 1 and 3. That is because, in addition to a yarn length corresponding to the width of the fabric, each warp requires an additional increment of yarn length for each passage through the central plane of the fabric. Warp yarns 1 and 3 pass traverse the thickness at the fabric twice for each pattern repeat and thus are characterized as having two interlacings per pattern repeat. Yarns 2 and 4 pass traverse the thickness of the fabric four times in each pattern and therefore are characterized as having four interlacings per repeat.
The inequality in the amount of sew-up or yarn lengths required for the various warp yarns of a pattern such as is shown in FIG. 1 do not present a significant problem for fabrics woven from relatively elastic mateials such as wool and other natural fibers. Likewise, in the use of twisted spun yarns spun from natural fibers, the so-called "plied" yarn, the yarns tend to compensate for unequal sew-up by yielding a degree of twist and thereby lengthening to some extent. However, no such "natural" compensation is possible with the use of monofilament or continuous filament/multifilament yarns which has come into vogue in recent years. When monofilament warp yarns are woven on a conventional loom in a pattern repeat as is shown in FIG. 1, there is no compensation for the difference in sew-up as between yarns 1 and 3 and yarns 2 and 4; rather, the problem is cumulative across the width of the fabric. In other words, as the weave pattern is repeated across the width of the (endless woven) fabric, warp yarns 1 and 3 accumulate slack, eventually to the point where yarns 1 and 3 can no longer be "shed", i.e. lifted above the pick. In addition to the problem presented to loom operatio, unequal sewup also causes defects in the fabric product. With unequal sewup, the tighter warp yarns, yarns 2 and 4 of the pattern of FIG. 1, tend to be straighter and locate more in the center of the fabric. Likewise, the more slack yarns, yarns 1 and 3 in the pattern of FIG. 1, tend to undulate in a loose crimp from surface to surface, with the result that the surface floats of yarns 1 and 3 will have a greater amplitude than the surface knuckles of yarns 2 and 4. Accordingly, unequal warp sew-up will result in the wearing of only 1/2 of the warp yarns on the machine side of the fabric and a deterioration of sheet support at the forming surface of the fabric.
One possible solution to the aforementioned problem of unequal warp sew-up would be to provide a loom with two warp-beam let-offs. However, such a solution to the problem represents an additional capital investment and complications in the loom operation, which factors make such a solution unattractive to many in the industry.
Thus, it is an object of the present invention to provide a papermakers' fabric with desirable sheet support, fiber retention and wear characteristics.
It is another object of the present invention to provide a fabric having such characteristics woven in a pattern with equal sew-up in the warp.
These and other objects, features and scope of applicability of the present invention will become apparent to those skilled in the art from a reading of the ensuing description in conjunction with the drawings.